The basic oxygen furnace (BOF) is a well-known steelmaking method used to produce most of the world's high-quality low carbon steel. The BOF process uses molten iron from a blast furnace as the primary iron source. Because of the nature of the blast furnace process, this molten blast furnace iron is saturated with dissolved carbon. In the BOF process, this molten iron is charged along with steel scrap into an open-top converter. A water-cooled lance is inserted through the top opening and oxygen is blown from the lance in multiple jets toward the molten metal. This oxygen burns the carbon contained in the molten iron, melting the scrap, and creating a bath of liquid steel.
The rate at which carbon is burned by the oxygen determines the productivity of the BOF process. When the partially refined liquid steel contains more than about 0.30 weight percent dissolved carbon, carbon oxidation occurs as quickly as oxygen can be blown through the lance to the liquid steel bath. Below about 0.30 weight percent carbon, however, the rate of carbon oxidation depends on the transport of dissolved carbon from the bulk of the steel bath to the area where the oxygen jet impacts the bath.
One problem with the conventional BOF process is that stirring and mixing in the bulk steel bath are relatively poor. As a result, below about 0.30 weight percent carbon, transport of carbon from the bulk bath to the reaction zone is slow and decarburization is inefficient. An increasing quantity of oxygen reacts with the metal rather than with the carbon as the carbon content decreases. Metallic oxidation results in loss to the slag of valuable elements such as iron and manganese. Such metallic oxidation is also costly because oxygen is consumed in excess of the steel making requirements, and the time to complete refining of the bath is increased. Furthermore, oxidation of other metallic alloying materials may lessen steel quality and increase the amount of costly re-alloying that is required. Excess metallic oxidation will also increase the temperature of the melt and the oxide content of the slag, both of which are detrimental to the refractory lining of the refining vessel. All of these problems reduce the efficiency and increase the cost of the BOF process.
Accordingly, it is an object of this invention to provide an improved top blowing process, such as an improved BOF process, for the production of low carbon steel.